The term "Boxing kangaroo" brought to my mind a cartoon image of a kangaroo with boxing gloves, but in the wild, these roo fights are less like boxing and more like brutal knife fights. Photographer Julian Robinson (aaardvaark on Flickr) observed:

Bouncing nimbly on powerful hind legs, sizing each other up, the two opponents launch sudden combinations, jabbing and grappling with lightning forelegs to gain the advantage, then lashing out with raking kicks – haymakers far more devastating than any man's. Each razorsharp hind claw has the potential to disembowel the other animal. [...] Although far from unfamiliar with Australia’s Eastern Grey Kangaroos, Julian Robinson was quite unprepared for the spectacle that was to unravel before his eyes. He stumbled upon the slugfest in 2006 [...]

The dangerous forepaw claws are clearly visible in the first two shots above. “They keep their heads out of range as far as they can, with good reason,” explains Julian, adding: “You don't see many roos standing up tall. These guys sure got as high as they could get.”

Slightly off topic, but only so much as it's not talking about boxing Kangaroos. Male Kangaroo's have been known to ejaculate at human females. Apparently human females smell the same as a female kangaroo on heat. No really. Fact.Ew.